In many important industrial chemical operations there exists a need to remove water and/or methanol from a mixture with other chemical components. Traditional methods of removing water or methanol from other components include fractional distillation, thermal evaporation, cryogenic dehydration, and chemical adsorption to name a few. Such methods have drawbacks such as requiring generally complex equipment and systems (e.g., distillation columns with associated pumps, heat exchangers and the like). They typically involve recirculating fluid and solvents in large volume relative to the product volume which adds to material cost as well as contributes to potential waste, safety and environmental protection concerns. Also these processes call for substantial energy input for heating and cooling of circulating fluids that further adds to the cost of operation. Chemical adsorption processes frequently operate cyclically and therefore additionally often utilize oversized and redundant adsorber units so that saturated units can be taken off-line for regeneration without interrupting production.
In some systems it is intrinsically difficult to separate water or methanol from the mixtures and conventional separation techniques are commensurately problematic. Mixtures in which there is a low relative volatility between water or methanol and other compounds or in which water or methanol form azeotropic compositions with the other compounds can be troublesome for traditional distillation methods to separate.
Membrane separation processes for segregating components of mixtures by selectively permeating individual components through a membrane are well known. An excellent recent survey of membrane pervaporation and vapor permeation processes is presented in “Pervaporation Comes of Age” N. Wynn, Chemical Engineering Progress, pp. 66-72, October, 2001. Very basically, in such processes one side of a selectively permeable membrane is contacted with the fluid mixture of components to be separated. A driving force, such as a pressure gradient across the membrane in the case of vapor permeation and concentration gradient in the case of pervaporation, causes preferentially permeating components to migrate through the membrane such that a permeate composition enriched in the faster permeating components develops on the other side of the membrane. A retentate composition on the feed mixture side of the membrane becomes depleted in the faster permeating components. With a vapor feed mixture fluid, this process is generally known as vapor permeation. When the feed fluid is in the liquid state, low pressure, usually vacuum, vaporizes the migrating components on the permeate side. This technique is known as pervaporation.
While offering a valuable alternative to other water and methanol removal methods, existing membrane separation technology also has limitations. Productivity is constrained by the separation characteristics of the membrane. It is a long standing problem in this field that permselective membranes usually have either high transmembrane flux or high selectivity but rarely both. The term “selectivity” means the ratio of permeability through a membrane of a faster permeating species divided by the permeability through the membrane of a slower permeating species. Thus the artisan must often choose a membrane material that sacrifices permeate flow rate to achieve an acceptable selectivity. Moreover many selectively permeable membrane compositions are not able to withstand exposure to temperatures of higher than about 100° C. The membrane deteriorates, weakens and ultimately fails. Hence operation of the separation process is limited to a maximum temperature that is safe for the membrane where a higher temperature would be more productive.
Certain membrane separation methods have been applied to drying of aqueous solutions. Membrane pervaporation of water from ethanol-water solutions of up to about 65 wt. % water using polyvinyl alcohol (PVA) active layer membrane composition is reported to provide a water permeance of less than about 5×10−4 kmol/m2-hr-mm Hg by D. Shah, D. Bhattacharyya, A. Ghorpade, and W. Mangum, “Pervaporation of pharmaceutical waste streams and synthetic mixtures using water selective membranes” Environmental Progress, 18(1), pp. 21-29, 1999, and M. S. Schehlmann, E. Wiedemann and R. N. Lichtenthaler, “Pervaporation and vapor permeation at the azeotropic point or in the vicinity of the LLE boundary phases of organic/aqueous mixtures” Journal of Membrane Science, 107(3), pp 277-282, 1995. Vapor permeation using a PVA active layer membrane provides even lower water permeance, i.e., less than about 1×10−4 kmol/m2-hr-mm Hg (Schehlmann et al. ibid). Others report significantly higher water permeance of as high as about 17×10−4 kmol/m2-hr-mm Hg using pervaporation with a PVA membrane at concentration of about 90 wt. % water (M. Wesslein, A. Heintz and R. N. Lichtenthaler, “Pervaporation of liquid mixtures through poly(vinyl alcohol) (pva) membranes. I. study of water containing binary systems with complete and partial miscibility” Journal of Membrane Science, 51(1-2) pp. 169-179, 1990). However, the water permeance decreases steadily as the concentration of water reduces to about 4×10−4 mol/m2-hr-mm Hg at about 20 wt. % water. All of the above studies report that below 10 wt. % water, the permeance of water is below about 2×10−4 kmol/m2-hr-mm Hg. Consequently conventional membrane separation of water from difficult-to-separate mixtures, while possible, occurs at impractically variable and low flow rates.
It is very desirable to have a membrane separation method of removing water and or methanol from fluid solutions that provides a practically high permeance through the membrane. It would also be beneficial to have such a membrane separation method in which the permeance is relatively uniform over a wide range of water concentration such that the permeance of water through the membrane does not reduce significantly as the concentration of water in the feed declines. It is further desired to have a membrane separation process that provides a substantial water permeance for feed mixtures of water concentration less than about 10 wt. %.
The commercial significance of a practical method to remove water from alcohol, especially ethyl alcohol (i.e., ethanol), has recently become acutely evident. Ethanol is prominently promoted as fuel for internal combustion engines because it is a renewable resource alternative to fossil fuels. It is also finding more immediate use as an additive in so-called reformulated gasoline as a replacement for methyl t-butyl ether (MTBE) to reduce automotive emissions and thus improve ambient air quality. Because ethanol is miscible with water, it easily takes up water from moisture in the air during storage and from liquid water in-leakage during ethanol transfer which are not uncommon in the industry. Water in excess of as little as 0.5 wt. % is recognized to render ethanol unsuitable for fuel applications.
Ethanol is likely to pick up excessive water during distribution via pipeline transfer unless scrupulous precautionary measures are taken. Such measures are beyond current practice standards and are extremely and impracticably difficult to implement in the foreseeable future. Consequently it has been largely necessary to transport dry ethanol in bulk via discrete shipping containers such as cargo tank trailers rather than pipelines. That system can be a major hindrance and costly inconvenience to the distribution of gasoline for automotive and other engine fuels. It would be very valuable to have a method of drying ethanol to less than 0.5 wt. % water for fuel applications and highly desirable for an ethanol drying method that can be adapted for use with a pipeline distribution system
Another area of commercial interest in which water contamination of fluid can be a significant impediment to productivity is that of providing hydrocarbon oil of various types for utilities such as mechanical lubrication, power transmission, combustion fuel and the like. Water contamination of oils that lubricates high speed machinery has a great tendency to form a foamy emulsion that drastically reduces lubrication ability and thus can cause premature wear of expensive and difficult to replace equipment. Even low concentrations of water in oil, i.e., in the range below about 2,500 parts per million by weight, can contribute to excessive acidic degradation of the oil, particularly at typically elevated equipment operating temperatures. Such degraded oil also has reduced lubrication capability. Moreover acid produced by the water-promoted degradation of oil can chemically corrode delicate or precision mechanical equipment that the oil is intended to protect. Ester-based hydraulic oil compositions are especially susceptible to acidic degradation because water can react with the esterified oil to form acid which catalyzes the reaction to produce still more acid and further oil degradation.
The water of contaminated oil can be removed by various effective conventional methods. Gross amounts of water are typically removed by coalescing filters and the like. The removal of water to the hundreds of parts per million range usually requires more extensive treatment methods than a simple mechanical filter can provide. However, those methods usually are carried out at oil regeneration disposal facilities which means that the contaminated oil must be removed from the equipment in which it is being used and typically shipped to the regeneration facility location.
Meanwhile the equipment is refilled with fresh oil. This system has a host of drawbacks including equipment downtime for removal and replacement of the contaminated oil, cost of purchase and storing stock of fresh oil to be ready for replacement when needed, handling and storage of contaminated oil waiting for shipment to regeneration, cost of shipping contaminated oil and the price of the service of regenerating or disposing of the contaminated oil. Costs attributed to the contamination and replacement of oil can be very high particularly when large scale equipment such as earthmoving machines, mining equipment, heavy military transportation equipment often located in remote regions are involved.
There is a great need for a method of decontaminating water-containing hydrocarbon oil that is relatively compact, light weight, low power-consuming and simple to operate in remote regions distant from sources of power, utilities and/or waste regeneration and disposal facilities. Such desirable method could operate in the field on wet oil taken from the oil-using equipment to obviate the cost of shipping in fresh oil and shipping out contaminated oil. It is highly desirable to have a method of removing water from oil capable of decontaminating wet hydrocarbon oil to very low concentrations of water in situ. That is, by removing the water during productive equipment operation. It is wanted to have such a water removal method that is small enough to mount on a piece of production equipment and which can draw the power needed for removing water from its host production equipment. For example, it would be helpful to have a hydraulic oil dehydration unit mounted on a bulldozer, surface mine excavator or ocean-crossing vessel. Such drying system would desirably avoid the need to shut down the operating equipment even temporarily by maintaining the concentration of water constantly within safe operational limits rather than waiting for the concentration to build up to an operating maximum value.
Chemical reaction productivity is another field in which removal via membrane of water or methanol has yet to be applied to fuller potential as described in the article by Wynn mentioned above The article points to esterification, acetalization and ketalization condensations as examples of reactions that are normally limited in ability to provide purer product at higher yield and with greater speed due to equilibrium considerations. Water byproduct present in the reaction mass shifts the equilibrium unfavorably. However, if water could be removed, the equilibrium would shift further toward the product side of the reaction equations. There is a need to remove water from these reaction compositions at high rate to promote productivity of equilibrium reactions. It is also occasionally needed to remove water from chemical reactions in the chemical and pharmaceutical field. In such systems traditional thermally motivated drying is detrimental to the chemicals and/or living organisms involved. There is a need to have a gentler and less thermally severe method of removing water from such reaction systems.